SiC substrate and SiC epitaxial wafer

ABSTRACT

In a SiC substrate of the present invention, in a case where a point 10 mm inside from an outer peripheral edge in a [11-20] direction from a center is defined as a first outer peripheral point and any point within a circle having a diameter of 10 mm from the center is defined as a first center point, the tensile stress of the first outer peripheral point in a &lt;1-100&gt; direction, which is a circumferential direction of the first outer peripheral point, is larger than the tensile stress of the first center point in the &lt;1-100&gt; direction, which is the same direction as the circumferential direction of the first outer peripheral point.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a SiC substrate and a SiC epitaxialwafer.

Description of Related Art

Silicon carbide (SiC) has a dielectric breakdown field one order ofmagnitude larger and a bandgap three times larger than silicon (Si). Inaddition, silicon carbide (SiC) has a property such as a thermalconductivity that is about three times as high as that of silicon (Si).Therefore, silicon carbide (SiC) is expected to be applied to powerdevices, high-frequency devices, high-temperature operation devices, andthe like. Thus, in recent years, SiC epitaxial wafers have come to beused for such semiconductor devices.

A SiC epitaxial wafer is obtained by stacking a SiC epitaxial layer on asurface of a SiC substrate cut out from a SiC ingot. Hereinafter, asubstrate before stacking the SiC epitaxial layer is referred to as theSiC substrate, and a substrate after stacking the SiC epitaxial layer isreferred to as the SIC epitaxial wafer.

The SiC epitaxial wafer may warp because it has the SiC epitaxial layeron one surface. The warpage of the SiC epitaxial wafer adversely affectsa process of a semiconductor device. For example, the warpage causesdefocus in photolithographic processing. In addition, the warpage causesa decrease in positional accuracy of a wafer during a transport process.Furthermore, the SiC epitaxial wafer may warp greatly due to oxide filmstacking or ion implantation during a semiconductor process.

Meanwhile, the SiC substrate is flat before stacking the SiC epitaxiallayer. Therefore, it is difficult to predict the warpage of the SiCepitaxial wafer or the warpage during the semiconductor process in thestate of the SiC substrate. For example, Patent Document 1 disclosesthat a difference in wavenumber shift amount of Raman scattered light isused to predict a value of warpage of a polished SiC single crystalproduct wafer before a polishing step is completed. Patent Document 2discloses a substrate in which the Raman spectrum is measured in athickness direction of the substrate and the stress distribution isreduced in the thickness direction. In addition, for example, PatentDocument 3 discloses that warpage of a SiC substrate is reduced byalleviating crystallographic stress.

Citation List

Patent Document

Patent Document 1:

Japanese Unexamined Patent Application, First Publication No. 2015-59073

Patent Document 2:

PCT International Publication No. WO2019/111507

Patent Document 3:

United States Patent Application, Publication No. 2021/0198804

Patent Document 4:

Japanese Unexamined Patent Application, First Publication No.2007-290880

SUMMARY OF THE INVENTION

In Patent Document 1 and Patent Document 2, an internal stress of asubstrate is evaluated using a Raman shift, but the Raman shift does notinclude information on a direction. In addition, Patent Document 1 toPatent Document 3 discloses that a stress decreases, but it isimpossible to sufficiently suppress warpage of the SiC epitaxial waferonly by reducing the stress. Further, Patent Document 4 discloses thatcracks on an ingot are suppressed by increasing the compressive stressin a peripheral direction of the ingot, however, it was impossible tosufficiently suppress the warpage of the SiC epitaxial wafer.

The present invention has been made in view of the above problems, andan object of the present invention is to provide a SiC substrate capableof suppressing warpage after performing a surface treatment such asstacking a SiC epitaxial layer, stacking an oxide film, performing ionimplantation, or the like.

The present inventors have found that it is possible to suppress thewarpage after performing the surface treatment such as the stacking ofthe SiC epitaxial layer or the like, by increasing the tensile stress ina circumferential direction in the vicinity of an outer periphery to belarger than the tensile stress in a circumferential direction in thevicinity of a center. That is, the present invention provides thefollowing means in order to solve the above problems.

(1) In a SiC substrate according to a first aspect, in a case where apoint 10 mm inside from an outer peripheral edge in a [11-20] directionfrom a center is defined as a first outer peripheral point and any pointwithin a circle having a diameter of 10 mm from the center is defined asa first center point, the tensile stress of the first outer peripheralpoint in a <1-100> direction, which is a circumferential direction ofthe first outer peripheral point, is larger than the tensile stress ofthe first center point in the <1-100> direction, which is the samedirection as the circumferential direction of the first outer peripheralpoint.

(2) In the SiC substrate according to the aspect described above, in acase where a point 10 mm inside from the outer peripheral edge in a[−1100] direction from the center is defined as a second outerperipheral point, the tensile stress of the second outer peripheralpoint in a <11-20> direction, which is the same direction as acircumferential direction of the second outer peripheral point, may belarger than the tensile stress of the first center point in the <11-20>direction, which is the same direction as the circumferential directionof the second outer peripheral point.

(3) In a SiC substrate according to a second aspect, in a case where apoint 10 mm inside from an outer peripheral edge in a [−1100] directionfrom a center is defined as a second outer peripheral point and anypoint within a circle having a diameter of 10 mm from the center isdefined as a first center point, the tensile stress of the second outerperipheral point in a <11-20> direction, which is a circumferentialdirection of the second outer peripheral point, is larger than thetensile stress of the first center point in the <11-20> direction, whichis the same direction as the circumferential direction of the secondouter peripheral point.

(4) In the SiC substrate according to the aspect described above, in acase where a point 10 mm inside from the outer peripheral edge in a[11-20] direction from the center is defined as a first outer peripheralpoint, the tensile stress of the first outer peripheral point in a<1-100> direction, which is the same direction as a circumferentialdirection of the first outer peripheral point, may be larger than thetensile stress of the first center point in the <1-100> direction, whichis the same direction as the circumferential direction of the firstouter peripheral point.

(5) In the SiC substrate according to the aspect described above, thetensile stress of the first outer peripheral point in thecircumferential direction of the first outer peripheral point may belarger than the tensile stress of the first center point acting in thesame direction as the circumferential direction of the first outerperipheral point by 10 MPa or more. In addition, the tensile stress ofthe second outer peripheral point in the circumferential direction maybe larger than the tensile stress of the first center point acting inthe same direction as the circumferential direction of the second outerperipheral point by 10 MPa or more.

(6) In the SiC substrate according to the aspect described above, thetensile stress of the first outer peripheral point in thecircumferential direction of the first outer peripheral point may belarger than the tensile stress of the first center point acting in thesame direction as the circumferential direction of the first outerperipheral point by 30 MPa or more. In addition, the tensile stress ofthe second outer peripheral point in the circumferential direction maybe larger than the tensile stress of the first center point acting inthe same direction as the circumferential direction of the second outerperipheral point by 30 MPa or more.

(7) The SiC substrate according to the aspect described above may have adiameter of 145 mm or more.

(8) The SiC substrate according to the aspect described above may have adiameter of 195 mm or more.

(9) In the SiC substrate according to the aspect described above, afirst surface may have a surface roughness (Ra) of 1 nm or less.

(10) The SiC substrate according to the aspect described above may havea warp of 50 μm or less.

(11) In the SiC substrate according to the aspect described above, in afirst surface, in a case where supports are positioned to overlap with acircumference on 7.5 mm inside from an outermost periphery and a planeconnecting parts overlapping with the supports when seen in a thicknessdirection is defined as a reference plane, a bow may be 30 μm or less.

(12) A SiC epitaxial wafer according to a third aspect includes the SiCsubstrate according to the aspect described above, and a SiC epitaxiallayer stacked on one surface of the SiC substrate.

(13) The SiC epitaxial wafer according to the aspect described above mayhave a warp of 50 μm or less.

(14) In the SiC epitaxial wafer according to the aspect described above,in a surface of the SiC epitaxial layer, in a case where supports arepositioned to overlap with a circumference on 7.5 mm inside from anoutermost periphery and a plane passing through parts overlapping withthe supports when seen in a thickness direction is defined as areference plane, a bow may be 30 μm or less.

The SiC substrate according to the aspect described above can suppressthe warpage after performing surface treatment such as stacking a SiCepitaxial layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a warpage of a SiC epitaxialwafer.

FIG. 2 is a plan view of a SiC substrate according to the presentembodiment.

FIG. 3 is a schematic diagram showing a method for measuring the tensilestress of a first outer peripheral point in a circumferential direction.

FIG. 4 is a schematic diagram showing a method of measuring the tensilestress of a second outer peripheral point in a circumferentialdirection.

FIG. 5 is a diagram schematically showing a method for evaluating theshape of a SiC substrate due to a warp.

FIG. 6 is a diagram schematically showing a method for evaluating theshape of a SiC substrate due to a bow.

FIG. 7 is a schematic diagram showing a sublimation method, which is anexample of a SiC ingot manufacturing apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a SiC substrate and the like according to the presentembodiment will be described in detail with appropriate reference to thedrawings. In the drawings used in the following description,characteristic parts may be enlarged for the sake of convenience inorder to make it easier to understand the characteristics of the presentembodiment, and dimensional ratios of constituent elements or the likemay differ from the actual ones. Materials, dimensions, and the likeexemplified in the following description are merely examples, and thepresent invention is not limited thereto and can be modified asappropriate without changing the gist of the invention.

First, warpage of a SiC epitaxial wafer 20 will be described. FIG. 1 isa schematic diagram showing the warpage of the SiC epitaxial wafer 20.The SiC epitaxial wafer 20 is obtained by stacking a SiC epitaxial layer11 on a first surface 10 a of a SiC substrate 10. The SiC epitaxialwafer 20 includes the SiC substrate 10 and the SiC epitaxial layer 11.

The SiC substrate 10 has no large warpage and is substantially flat. Theterm “substantially flat” means that there is no part that risessignificantly when placed on a flat surface.

The SiC epitaxial layer 11 is stacked on the SiC substrate 10 in orderto obtain high-quality SiC on which a device can operate. In addition,mechanical processing such as polishing is often performed beforestacking the SiC epitaxial layer 11. In this case, a work-affected layeris formed on the first surface 10 a of the SiC substrate 10. The SiCepitaxial wafer 20 may warp, in a case where the SiC epitaxial layer 11is stacked or the work-affected layer is formed on one surface of theSiC substrate 10.

“First Embodiment”

FIG. 2 shows the SiC substrate 10 according to the present embodiment.The SiC substrate 10 is formed of SiC. A polytype of the SiC substrate10 is not particularly limited and may be any of 2H, 3C, 4H, and 6H. TheSiC substrate 10 is, for example, 4H-SiC.

A planar view shape of the SiC substrate 10 is substantially circular.The SiC substrate 10 may have an orientation flat OF or a notch fordetermining a direction of a crystal axis. A diameter of the SiCsubstrate 10 is, for example, 145 mm or more, and preferably 195 mm ormore. The larger the diameter of the SiC substrate 10, the larger theabsolute amount of warpage even with the same curvature. A SiC epitaxialwafer with large warpage has a great influence on subsequent steps, andit is required to suppress the warpage. In other words, the presentinvention is more effective as it is applied to the SiC substrate 10having a large diameter.

In the SiC substrate 10 according to the present embodiment, the tensilestress in a <1-100> direction, which is a circumferential direction of afirst outer peripheral point 1, is larger than the tensile stress of afirst center point 2 in the <1-100> direction, which is the samedirection as the circumferential direction of the first outer peripheralpoint 1. The tensile stress of the first outer peripheral point 1 in thecircumferential direction is larger than the tensile stress of the firstcenter point 2 acting in the same direction as the circumferentialdirection of the first outer peripheral point 1 preferably by 10 MPa ormore and more preferably by 30 MPa or more.

The first outer peripheral point 1 is on an outer peripheral part 5 10mm inside from an outer peripheral edge of the SiC substrate 10. Thefirst outer peripheral point 1 is a point on the outer peripheral part 5in a [11-20] direction from a center of the SiC substrate 10. The firstcenter point 2 is any point within a center part 6. The center part 6 isa region within a circle having a diameter of 10 mm from the center ofthe SiC substrate 10. The first center point 2 coincides with the centerof the SiC substrate 10, for example.

Here, <> and [] are used as bracket notations indicating directions ofthe Miller index. <1-100> includes [−1100] due to the symmetry of acrystal direction. <11-20>includes [11-20] due to the symmetry of thecrystal direction.

The tensile stress is calculated as the product of a strain c and aYoung's modulus. The strain ε is obtained by (a₀−a)/a₀. a₀ is areference lattice constant. a₀ is approximately 3.08 Å in a case of4H-SiC. a is a lattice constant obtained by an X-ray diffraction method(XRD). A direction of the stress is obtained from a direction ofincident X-rays in the X-ray diffraction. In the present invention,tension is considered a positive value, and compression is considered anegative value. When discussing the magnitude of stress, the magnitudeis defined by an absolute value. As the lattice constant a becomessmaller than the reference lattice constant a₀, the strain ε increases.As a result, the tensile stress increases.

FIG. 3 is a schematic diagram showing a method for measuring the tensilestress of the first outer peripheral point 1 in the circumferentialdirection. The circumferential direction of the first outer peripheralpoint 1 is a direction orthogonal to a line segment connecting thecenter of the SiC substrate 10 and the first outer peripheral point 1(hereinafter, referred to as a first direction). The first direction isthe <1-100> direction. In a case of measuring the tensile stress of thefirst outer peripheral point 1 in the circumferential direction, X-raysare irradiated from the first direction. By emitting the X-rays to theSiC substrate 10 from this circumferential direction, the latticeconstant a of the first outer peripheral point 1 in the circumferentialdirection is obtained. Then, using this lattice constant a, the stressin the circumferential direction of the first outer peripheral point 1is obtained from the above equation. In a case where the actuallymeasured lattice constant a is smaller than the reference latticeconstant a₀, it is assumed that the tensile stress is acting.

The tensile stress of the first center point 2 acting in the samedirection as the circumferential direction of the first outer peripheralpoint 1 is obtained by emitting the X-rays to the first center point 2by the same method for the first outer peripheral point 1. The samedirection as the circumferential direction of the first outer peripheralpoint 1 is the first direction described above. The first outerperipheral point 1 and the first center point 2 are compared inmagnitude of the tensile stress acting in the same direction (the firstdirection).

In a case where the tensile stress in the circumferential direction ofthe first outer peripheral point 1 is greater than the tensile stress ofthe first center point 2 acting in the same direction as thecircumferential direction of the first outer peripheral point 1, the SiCepitaxial wafer 20 is less likely to warp after stacking the SiCepitaxial layer 11. It is considered that this is because, as a strongtensile stress is applied to the circumferential direction of the firstouter peripheral point 1, a force for widening the SiC epitaxial wafer20 outward acts on the SiC epitaxial wafer 20.

In addition, in the SiC substrate 10 according to the presentembodiment, the tensile stress of the second outer peripheral point 3 inthe circumferential direction is preferably greater than the tensilestress of the first center point 2 acting in the same direction as thecircumferential direction of the second outer peripheral point 3. Thetensile stress in the circumferential direction of the second outerperipheral point 3 is larger than the tensile stress of the first centerpoint 2 acting in the same direction as the circumferential direction ofthe second outer peripheral point 3 preferably by 10 MPa or more andmore preferably by 30 MPa or more.

The second outer peripheral point 3 is on an outer peripheral part 5 10mm inside from an outer peripheral edge of the SiC substrate 10. Thesecond outer peripheral point 3 is a point on the outer peripheral part5 in a [−1100] direction from a center of the SiC substrate 10.

FIG. 4 is a schematic diagram showing a method for measuring the tensilestress of the second outer peripheral point 3 in the circumferentialdirection. The circumferential direction of the second outer peripheralpoint 3 is a direction orthogonal to a line segment connecting thecenter of the SiC substrate 10 and the second outer peripheral point 3(hereinafter, referred to as a second direction). The second directionis the <11-20> direction. In a case of measuring the tensile stress ofthe second outer peripheral point 3 in the circumferential direction,X-rays are irradiated from the second direction. By emitting the X-raysto the SiC substrate 10 from this circumferential direction, the latticeconstant a of the second outer peripheral point 3 in the circumferentialdirection is obtained. Then, using this lattice constant a, the tensilestress in the circumferential direction of the second outer peripheralpoint 3 is obtained from the above equation.

In a case of comparing the tensile stress of the second outer peripheralpoint 3 in the circumferential direction with the tensile stress of thefirst center point 2 acting in the same direction as the circumferentialdirection of the second outer peripheral point 3, the tensile stress ofthe first center point 2 in the <11-20> direction which is the samedirection as the circumferential direction of the second outerperipheral point 3 is obtained. The tensile stress of the first centerpoint 2 acting in the same direction as the circumferential direction ofthe second outer peripheral point 3 is obtained by emitting the X-raysto the first center point 2 by the same method for the second outerperipheral point 3. The same direction as the circumferential directionof the second outer peripheral point 3 is the second direction describedabove.

In a case where the tensile stress in the circumferential direction ofthe second outer peripheral point 3 is greater than the tensile stressof the first center point 2 acting in the same direction as thecircumferential direction of the second outer peripheral point 3, theSiC epitaxial wafer 20 is further less likely to warp after stacking theSiC epitaxial layer 11. It is considered that this is because a forcefor widening the SiC epitaxial wafer 20 outward acts in differentdirections in plane of the SiC epitaxial wafer 20.

In addition, in the SiC substrate 10 according to the presentembodiment, it is preferable that the tensile stress in thecircumferential direction is larger than the tensile stress of the firstcenter point 2 at any position on the outer peripheral part 5. Here, thetensile stress of the first center point 2 is the tensile stress actingin the same direction as the circumferential direction at a measurementpoint. In addition, the average tensile stress applied to a region on anouter side of the outer peripheral part 5 is preferably larger than theaverage tensile stress applied to the center part 6. Here, the averagetensile stress is, for example, the average value of tensile stressesmeasured at five different points within the region.

A surface of the SiC substrate 10 is often ground. A surface roughness(Ra) of the first surface 10 a of the SiC substrate 10 is preferably,for example, 1 nm or less. The first surface 10 a is, for example, asurface on which the SiC epitaxial layer 11 is stacked.

Both of the first surface 10 a and a second surface 10 b of the SiCsubstrate 10 may be ground. The first surface 10 a is, for example, a Sisurface, and the second surface 10 b is, for example, a C surface. Therelationship between the first surface 10 a and the second surface 10 bmay be reversed. Both the first surface 10 a and the second surface 10 bmay be mirror-finished mirror surfaces with residual scratches or thelike, or may be CMP-processed surfaces subjected to chemical mechanicalPolishing (CMP), and the polishing degree may be different between thefirst surface 10 a and the second surface 10 b. The work-affected layeris formed on the mirror surface with residual scratches or the like, andalmost no work-affected layer is formed on the CMP-processed surface.The work-affected layer is a part that has been damaged by processingand has a collapsed crystal structure.

For example, in a case where the first surface 10 a is a mirror-groundsurface and the second surface 10 b is a CMP-processed surface, theTwyman effect occurs in the SiC substrate 10 due to a difference insurface state between the two surfaces. The Twyman effect is aphenomenon in which, in a case where there is a difference occurs inresidual stress on both surfaces of a substrate, a force acts tocompensate for the difference in stress on both surfaces. The Twymaneffect may cause the warpage of the SiC epitaxial wafer 20. That is, thepresent invention is more effective as it is applied to the SiCsubstrate 10 in which the surface states of the first surface 10 a andthe second surface 10 b are different.

The SiC substrate 10 according to the present embodiment preferably hasa warp of 50 μm or less, and more preferably 30 μm or less. In a casewhere the SiC substrate 10 satisfying the relationship of the tensilestress with the warp of 50 μm or less is used, it is possible tosufficiently reduce the warpage of the SiC epitaxial wafer 20.Therefore, the SiC epitaxial wafer 20 can be prevented from beinglowered in accuracy during transportation, and can be properly focusedeven in a fine lithography process.

FIG. 5 is a diagram schematically showing a method for evaluating theshape (deformation) of a SiC substrate due to a warp. The warp is adistance between a highest point hp and a lowest point lp of the firstsurface 10 a in a thickness direction. It is determined that the largerthe warp is, the more the SiC substrate 10 is deformed. First, the SiCsubstrate 10 is placed on three support points placed on a flat surfaceF. A virtual plane Slp which passes through the lowest point lp of thefirst surface 10 a and is parallel to the flat surface F and a virtualplane Shp which passes through the highest point hp of the first surface10 a and is parallel to the flat surface F are obtained. The warp isobtained as a distance between the virtual plane Slp and the virtualplane Shp in a height direction. The height direction is a directionorthogonal to the flat surface F and away from the flat surface F.

The SiC substrate 10 according to the present embodiment preferably hasa bow of 30 μm or less, and more preferably 10 μm or less. In addition,the bow is preferably—30 μm or more. In a case where the SiC substrate10 satisfying the relationship of the tensile stress with an absolutevalue of the bow of 30 μm or less is used, it is possible tosufficiently reduce the warpage of the SiC epitaxial wafer 20.Therefore, the SiC epitaxial wafer 20 can be prevented from beinglowered in accuracy during transportation, and can be properly focusedeven in a fine lithography process.

FIG. 6 is a diagram schematically showing a method for evaluating theshape (deformation) of a SiC substrate due to a bow. The bow is aposition of a center c of the SiC substrate 10 with respect to areference plane Sr in the height direction. In other words, the bow is asigned distance of the center c of the SiC substrate 10 from thereference plane Sr. The reference plane Sr is a plane connecting pointssp of the first surface 10 a overlapping each of the plurality ofsupports, when seen in the thickness direction. The plurality ofsupports are arranged, for example, at positions overlapping acircumference of the SiC substrate 10 7.5 mm inside from the outerperipheral edge. For example, the SiC substrate 10 is supported by threesupports. Each of the three supports is positioned 3-fold symmetricallyabout the center of the SiC substrate 10 supported by the support. Thereference plane Sr is, for example, a 3-point reference flat plane. Itis determined that the larger the absolute value of the bow is, the morethe SiC substrate 10 is deformed. First, the SiC substrate 10 is placedon three support points placed on a flat surface F. The reference planeSr is obtained by connecting three points sp of the first surface 10 aon a support point when seen in the thickness direction. Then, thereference plane Sr is defined as 0, a direction away from the flatsurface F with reference to the reference plane Sr is defined as +, anda direction approaching the flat surface F with reference to thereference plane Sr is defined as −. The bow is obtained as a position ofthe center c of the first surface 10 a with respect to the referenceplane Sr in the height direction. In other words, the bow is obtained asa signed distance of the center c of the first surface 10 a from thereference plane Sr.

In addition, the SiC epitaxial wafer 20 after stacking the SiC epitaxiallayer 11 also has the warp of preferably 50 μm or less and morepreferably 30 μm or less. Further, the SiC epitaxial wafer afterstacking the SiC epitaxial layer 11 also has the bow of preferably 30 μmor less, more preferably 10 μm or less, and preferably −30 μm or more.The reference plane for measuring the bow of the SiC epitaxial wafer 20is a plane connecting points of the surface of the SiC epitaxial layer11 overlapping each of the plurality of supports when seen in thethickness direction. The positions of the plurality of supports are thesame as the positions for measuring the bow of the SiC substrate 10.First, the SiC epitaxial wafer 20 is placed on three support pointsplaced on the flat surface F. Three points on the surface of the SiCepitaxial layer 11 on the support point when seen in the thicknessdirection are connected, and the reference plane for measuring the bowof the SiC epitaxial wafer 20 is obtained. The bow is obtained as theposition of the center of the surface of the SiC epitaxial layer 11 withrespect to the reference plane in the height direction. In other words,the bow is obtained as a signed distance of the center of the surface ofthe SiC epitaxial layer 11 from the reference plane.

Next, an example of a method for manufacturing the SiC substrate 10according to the present embodiment will be described. The SiC substrate10 is obtained by slicing a SiC ingot. The SiC ingot is obtained, forexample, by a sublimation method.

FIG. 7 is a schematic diagram showing a sublimation method, which is anexample of a SiC ingot manufacturing apparatus 30. In FIG. 7 , adirection orthogonal to a surface of a pedestal 32 is defined as a zdirection, one direction orthogonal to the z-direction is defined as anx direction, and a direction orthogonal to the z direction and the xdirection is defined as a y direction.

The sublimation method is a method for disposing a seed crystal 33formed of SiC single crystal on the pedestal 32 disposed in a crucible31 formed of graphite, heating the crucible 31 to supply a sublimationgas sublimated from a raw material powder 34 in the crucible 31 to theseed crystal 33, and growing the seed crystal 33 into a larger SiC ingot35. The heating of the crucible 31 is performed by a coil 36, forexample.

By controlling crystal growth conditions in the sublimation method, thetensile stress applied to an inner part of the SiC substrate 10 obtainedfrom the SiC ingot 35 can be controlled.

For example, in a case of performing c plane growth of the SiC ingot 35,a temperature of a center part of a crystal growth surface and atemperature of an outer peripheral part are controlled. The crystalgrowth surface is a surface during a growth process of the crystal. Forexample, in a case of performing the c plane growth of the SiC ingot 35,the temperature of the outer peripheral part is reduced to be lower thanthe temperature of the center part of the crystal growth surface. Inaddition, the crystal growth is performed so that a difference in growthspeed between the center and the outer periphery in an xy plane is 0.001mm/h or more and 0.05 mm/h or less. Here, the growth speed of in thecenter in the xy plane is reduced to be slower than the growth speed ofthe outer periphery. The growth speed is changed by changing thetemperature of the crystal growth surface.

The temperature of the crystal growth surface can be adjusted bycontrolling a position of a heating center of the crucible 31 due to thecoil 36 in the z direction. For example, the position of the heatingcenter of the crucible 31 in the z direction can be changed by changingthe position of the coil 36 in the z direction. The position of theheating center of the crucible 31 in the z direction and the position ofthe crystal growth surface in the z direction are controlled to beseparated by 0.5 mm/h. Here, the position of the heating center of thecrucible 31 in the z direction is controlled to be on a lower side (aside of the raw material powder 34) with respect to the position of thecrystal growth surface in the z direction.

Next, the SiC ingot manufactured under such conditions is processed intothe SiC substrate 10. In a general processing method, the stress appliedto the single crystal changes depending on a state of the SiC ingot anda state of the SiC substrate. For example, in a shaping step, whenprocessing the SiC ingot having a diameter of 180 mm into the SiCsubstrate having a diameter of 150 mm, it is necessary to reduce thediameter. In addition, for example, in a multi-wire cutting step,undulations occur on the surface, and it is necessary to remove theundulations. Through such a step, for example, a part of the SiC ingothaving a large stress may be removed or a shape of a crystal latticesurface is changed, the stress of the state of the SiC ingot may bereleased in the state of the SiC substrate, and a SiC substrate having alarge tensile stress on the outer peripheral part cannot be obtained. Inorder to obtain a SiC substrate having a large tensile stress on theouter peripheral part, it is necessary to perform processing so that thestress applied to the single crystal in the state of the ingot istransferred to the state of the substrate.

For example, after performing damage-free processing on one surface ofthe SiC ingot, it is cut with a single wire saw, and thedamage-free-processed surface is adsorbed to further perform thedamage-free processing on the cut surface. By performing the damage-freeprocessing on both surfaces of the SiC substrate 10, a part of thetensile stress generated in the state of the SiC ingot is transferred tothe substrate. The damage-free processing is, for example, CMPprocessing. By performing substrate processing to remain a latticesurface shape of the state of the SiC ingot, the SiC substrate 10 havinga large tensile stress can be manufactured without releasing the stressof the SiC ingot. After that, by performing a shaping step for adjustingthe diameter, the SiC substrate 10 having a large tensile stress can beobtained.

As described above, the SiC substrate 10 according to the firstembodiment is less likely to warp even after the SiC epitaxial layer 11is stacked. It is considered that this is because a force for wideningthe SiC epitaxial wafer 20 outward acts by intentionally increasing thetensile stress in a peripheral direction on an outer side of the SiCsubstrate 10.

“Second Embodiment”

In the SiC substrate 10 according to a second embodiment, the tensilestress in the <11-20> direction, which is a circumferential direction ofthe second outer peripheral point 3, is larger than the tensile stressof the first center point 2 acting in the <11-20>direction, which is thesame direction as the circumferential direction of the second outerperipheral point 3. The SiC substrate 10 of the second embodiment is thesame as the SiC substrate 10 of the first embodiment, except thatmeasurement points for defining the state of the SiC substrate 10 aredifferent. For example, the preferred ranges of the warp, the bow, thediameter, the surface roughness, and the like of the SiC substrate 10according to the second embodiment are the same as those of the SiCsubstrate 10 according to the first embodiment.

The tensile stress of the second outer peripheral point 3 in thecircumferential direction is larger than the tensile stress of the firstcenter point 2 acting in the same direction as the circumferentialdirection of the second outer peripheral point 3 preferably by 10 MPa ormore and more preferably by 30 MPa or more.

In addition, the tensile stress in a <1-100> direction, which is acircumferential direction of a first outer peripheral point 1, ispreferably larger than the tensile stress of a first center point 2 inthe <1-100> direction, which is the same direction as thecircumferential direction of the first outer peripheral point 1. Thetensile stress in the circumferential direction of the first outerperipheral point 1 is larger than the tensile stress of the first centerpoint 2 acting in the same direction as the circumferential direction ofthe first outer peripheral point preferably by 10 MPa or more and morepreferably by 30 MPa or more.

The SiC substrate 10 according to the second embodiment has the sameeffect as the SiC substrate 10 according to the first embodiment.

Although the preferred embodiments of the present invention have beendescribed in detail above, the present invention is not limited tospecific embodiments, and can be variously modified and changed withinthe gist of the present invention described in claims.

EXAMPLES

(Example 1)

A warpage in a case where a SiC epitaxial layer was stacked on a surfaceof a SiC substrate was obtained by simulation. The simulation wasperformed using the finite element method simulator ANSYS. It wasseparately confirmed that the simulation using the finite element methodsimulator ANSYS matched a result of an actually manufactured product.

The simulation was performed in the following procedure. First, physicalproperty values of the SiC substrate and a surface layer having adifferent stress were set.

The physical property values to be set were a plate thickness of the SiCsubstrate, a film thickness of the surface layer, a Young's modulus, anda Poisson's ratio. The plate thickness of the SiC substrate was set to350 μm. A diameter of the SiC substrate was 150 mm. A warp of the SiCsubstrate was set to 0 μm. The Young's modulus of the SiC substrate wasset to 480 GPa and the Poisson's ratio thereof was set to 0.20. The filmthickness of the surface layer was set to 10 μm. Here, considering acase where the stress was generated in the surface layer by ionimplantation, the same values as those of the SiC substrate were usedfor the Young's modulus and Poisson's ratio of the surface layer.

Next, a stress distribution of the SiC substrate and a stress of thesurface layer were set. First, the stress distribution of the SiCsubstrate was set. The tensile stress of the first outer peripheralpoint 1 of the SiC substrate was set to be larger than the tensilestress of the first center point 2 by 40 MPa. That is, the difference instress between the first outer peripheral point 1 and the first centerpoint 2 was set to 40 MPa, and the first outer peripheral point 1 wasset to be applied with stronger tensile stress than that of the firstcenter point. The stress of 60 MPa was applied to the entire surfacelayer.

The simulation was performed under the above conditions to obtain thewarpage of the SiC substrate with the surface layer. The warpage wasevaluated by a warp. The warpage (the warp) of Example 1 was 47 μm. Thewarpage of the SiC substrate with the surface layer can be regarded aswarpage of an epitaxial wafer by regarding the surface layer as anepitaxial layer. In a case where the surface layer was an epitaxiallayer, the magnitude of warp changes depending on a stress differencethat depends on a film thickness of the epitaxial layer or a differencein impurity concentration. However, it was confirmed that there was acorrelation with the obtained warpage of the SiC substrate with thesurface layer.

(Example 2)

Example 2 was different from Example 1 in that the tensile stress of thefirst outer peripheral point 1 of the SiC substrate was set to be largerthan the tensile stress of the first center point 2 by 20 MPa. That is,a difference in stress between the first outer peripheral point 1 andthe first center point 2 was set to 20 MPa, and the first outerperipheral point 1 was set to be applied with stronger tensile stressthan that of the first center point. Other parameters were set as thesame as in Example 1, and the warpage (the warp) of the SiC substratewith the surface layer was obtained by the simulation in the same manneras in Example 1. The warpage (the warp) of Example 2 was 78 μm.

(Comparative Example 1)

Comparative Example 1 was different from Example 1 in that the tensilestress of the first outer peripheral point 1 of the SiC substrate wasset to be the same as the tensile stress of the first center point 2.That is, the difference in stress between the first outer peripheralpoint 1 and the first center point 2 was set to 0 MPa, and the firstouter peripheral point 1 was set to be applied with the same stress asthat of the first center point 2. Other parameters were set as the sameas in Example 1, and the warpage (the warp) of the SiC substrate withthe surface layer was obtained by the simulation in the same manner asin Example 1. The warpage (the warp) of Comparative Example 1 was 116μm.

(Comparative Example 2)

Comparative Example 2 was different from Example 1 in that the tensilestress of the first center point 2 of the SiC substrate was set to belarger than the tensile stress of the first outer peripheral point 1 by20 MPa. That is, a difference in stress between the first outerperipheral point 1 and the first center point 2 was set to −20 MPa, andthe first center point 2 was set to be applied with stronger tensilestress than that of the first outer peripheral point 1. That is, thefirst outer peripheral point 1 was applied with a compressive stressthan the first center point 2. Other parameters were set as the same asin Example 1, and the warpage (the warp) of the SiC substrate with thesurface layer was obtained by the simulation in the same manner as inExample 1. The warpage (the warp) of Comparative Example 2 was 189 μm.

The results of Examples 1 and 2 and Comparative Examples 1 and 2 weresummarized in Table 1.

TABLE 1 Difference in stress (first outer peripheral point − firstcenter point) Warpage Example 1  40 MPa  47 μm Example 2  20 MPa  78 μmComparative  0 MPa 116 μm Example 1 Comparative −20 MPa 189 μm Example 2

In Examples 1 and 2, in which a larger tensile stress acts on the outerperipheral part than in the center part, the warpage of the SiCsubstrate with the surface layer was small, compared to ComparativeExample 1 in which the tensile stress did not act on the outerperipheral part compared to the center part and Comparative Example 2 inwhich the compressive stress acts on the outer peripheral part. That is,in the substrate having a tensile stress in the outer peripheral partcompared to the center part, the warpage of the epitaxial wafer during asemiconductor process can be reduced, compared to the SiC substrate inwhich the tensile stress does not act on the outer peripheral partcompared to the center part and the SiC substrate in which thecompressive stress acts on the outer peripheral part.

EXPLANATION OF REFERENCES

-   -   1 first outer peripheral point    -   2 first center point    -   3 second outer peripheral point    -   5 outer peripheral part    -   6 center part    -   10 SiC substrate    -   10 a first surface    -   10 b second surface    -   11 SiC epitaxial layer    -   20 SiC epitaxial wafer    -   hp highest point    -   lp lowest point    -   sp support point    -   Shp, Slp virtual plane    -   Sr reference plane

What is claimed is:
 1. A SiC substrate, wherein, in a case where a point10 mm inside from an outer peripheral edge in a [11-20] direction from acenter is defined as a first outer peripheral point and any point withina circle having a diameter of 10 mm from the center is defined as afirst center point, a tensile stress of the first outer peripheral pointin a <1-100> direction, which is a circumferential direction of thefirst outer peripheral point, is larger than a tensile stress of thefirst center point in the <1-100> direction, which is the same directionas the circumferential direction of the first outer peripheral point,the tensile stress of the first outer peripheral point in thecircumferential direction of the first outer peripheral point is largerthan the tensile stress of the first center point acting in the samedirection as the circumferential direction of the first outer peripheralpoint by 10 MPa or more, and the tensile stress is calculated as aproduct of a strain and a Young's modulus, the strain is obtained by(a₀−a)/a₀, a₀ is a reference lattice constant, and a is a latticeconstant obtained by an X-ray diffraction method.
 2. The SiC substrateaccording to claim 1, wherein, in a case where a point 10 mm inside fromthe outer peripheral edge in a [−1100]direction from the center isdefined as a second outer peripheral point, a tensile stress of thesecond outer peripheral point in a <11-20> direction, which is the samedirection as a circumferential direction of the second outer peripheralpoint, is larger than a tensile stress of the first center point in the<11-20> direction, which is the same direction as the circumferentialdirection of the second outer peripheral point.
 3. A SiC substrate,wherein, in a case where a point 10 mm inside from an outer peripheraledge in a [−1100]direction from a center is defined as a second outerperipheral point and any point within a circle having a diameter of 10mm from the center is defined as a first center point, a tensile stressof the second outer peripheral point in a <11-20> direction, which is acircumferential direction of the second outer peripheral point, islarger than a tensile stress of the first center point in the <11-20>direction, which is the same direction as the circumferential directionof the second outer peripheral point, the tensile stress of the secondouter peripheral point in the circumferential direction of the secondouter peripheral point is larger than the tensile stress of the firstcenter point acting in the same direction as the circumferentialdirection of the second outer peripheral point by 10 MPa or more, andthe tensile stress is calculated as a product of a strain and a Young'smodulus, the strain is obtained by (a₀−a)/a₀, a₀ is a reference latticeconstant, and a is a lattice constant obtained by an X-ray diffractionmethod.
 4. The SiC substrate according to claim 3, wherein, in a casewhere a point 10 mm inside from the outer peripheral edge in a[11-20]direction from the center is defined as a first outer peripheralpoint, a tensile stress of the first outer peripheral point in a <1-100>direction, which is the same direction as a circumferential direction ofthe first outer peripheral point, is larger than a tensile stress of thefirst center point in the <1-100> direction, which is the same directionas the circumferential direction of the first outer peripheral point. 5.The SiC substrate according to claim 1, wherein the tensile stress ofthe first outer peripheral point in the circumferential direction of thefirst outer peripheral point is larger than the tensile stress of thefirst center point acting in the same direction as the circumferentialdirection of the first outer peripheral point by 30 MPa or more.
 6. TheSiC substrate according to claim 2, wherein the tensile stress of thesecond outer peripheral point in the circumferential direction of thesecond outer peripheral point is larger than the tensile stress of thefirst center point acting in the same direction as the circumferentialdirection of the second outer peripheral point by 10 MPa or more.
 7. TheSiC substrate according to claim 1, wherein a diameter is 145 mm ormore.
 8. The SiC substrate according to claim 1, wherein the diameter is195 mm or more.
 9. The SiC substrate according to claim 1, wherein afirst surface has a surface roughness (Ra) of 1 nm or less.
 10. The SiCsubstrate according to claim 1, wherein a warp is 50 μm or less.
 11. TheSiC substrate according to claim 1, wherein, in a first surface, in acase where supports are positioned to overlap with a circumference on7.5 mm inside from an outermost periphery and a plane connecting partsoverlapping with the supports when seen in a thickness direction isdefined as a reference plane, a bow is 30 μm or less.
 12. The SiCsubstrate according to claim 2, wherein the tensile stress of the secondouter peripheral point in the circumferential direction of the secondouter peripheral point is larger than the tensile stress of the firstcenter point acting in the same direction as the circumferentialdirection of the second outer peripheral point by 30 MPa or more. 13.The SiC substrate according to claim 2, wherein the diameter is 195 mmor more.
 14. The SiC substrate according to claim 3, wherein thediameter is 195 mm or more.
 15. The SiC substrate according to claim 4,wherein the diameter is 195 mm or more.
 16. The SiC substrate accordingto claim 5, wherein the diameter is 195 mm or more.
 17. The SiCsubstrate according to claim 6, wherein the diameter is 195 mm or more.18. The SiC substrate according to claim 9, wherein the diameter is 195mm or more.
 19. The SiC substrate according to claim 10, wherein thediameter is 195 mm or more.
 20. The SiC substrate according to claim 11,wherein the diameter is 195 mm or more.
 21. The SiC substrate accordingto claim 11, wherein the diameter is 195 mm or more.